@BraveDude8 i've seen this comment in other threads and it has got me curious if there is a language difference or something. To me a promo not just in manga but in most areas is meant to be a taste or a tease to get you to get more of whatever product. The deli gives you a taste of pasta salad to promote the pastas salad that is about to expire, uh, I mean they want to sell, they are hoping you will buy more, a pound, half pound or some other amount. Even if they give you a pound it is usually with the idea that you will start coming back for more. A promo is kind of test marketing to see if there is demand for more or if they can create a demand for more.
I view a oneshot product as complete product that you aren't going to come back for more of in a short time. Things like cars, appliances, a self-contained book in that you might look for more books to read but that story is finished. A oneshot is self-contained, the story is completed within that chapter.
To me if it is an adaptation of a novel that has more than one chapter it is likely not a oneshot but a promo.
If it is an incomplete story vs a completed story then it makes me think of test marketing with the hope that there is demand for more, so promo.